Logging-While-Drilling (LWD) instruments are widely used in oil and gas drilling and formation evaluation. They collect information such as formation resistivity, gamma ray, neutron porosity, borehole caliper, well inclination during the drilling process and transmit the real-time information to the surface. Such information is essential in real-time control of the bit so that the well trajectory closely follows the planned geometric trajectory and can also be adjusted in response to changes in the formation. However, since conventional LWD instruments are installed at some distance behind the drill bit, e.g., 10 meters or more, they cannot measure the conditions at the drill bit. Delays in obtaining the formation information results in delays in adjusting the bit orientation, which lowers drilling efficiency.
Directional drilling requires geological information at or close to the bit to effectively identify formation boundaries and steer the well trajectory in a way that maximizes its exposure in the pay zone. Near-bit measurement of resistivity, gamma ray, and well inclination, etc. become more important in this situation. However, to pack many LWD instruments near the bit can be challenging due to the limited space available near the drill bit. For example, in azimuthal resistivity measurement, in order to minimize the size of logging tool, guard electrodes are often omitted from the logging device so that a substantial portion of the electric current travels into the drilling mud. The measurement current, after flowing from the central electrode, quickly dissipates into the drilling fluid as well as the shoulder bed so that the vertical resolution of resistivity is very low and the resistivity measurement is greatly skewed by the borehole effects. On the other hand, some laterologging tools employ multiple pairs of monitor electrodes and guard electrodes in order to focus the measurement current. Nevertheless, more electrodes require more complex circuitry and lower the system reliability.
LWD instruments may be installed on the bent housing of the downhole drilling motor or between the bent housing and the drill collar below the bent housing. However, this would change the structure around the bend, which in turn affects the build angle and reduces the strength of the drilling tool. Such a drilling tool also needs to satisfy a certain well inclination. Although these problems may be alleviated by installing the resistivity logging tool between the downhole drill motor and the bent housing, the measurement is further away from the bit. Furthermore, directional drilling requires correlating resistivity with gamma rays and other azimuthal measurements to gather azimuthal resistivity and identify formation boundaries.
Accordingly, there is a need for tools and methods for measuring and transmitting information at or near the drill bit.